Remove duplicate code from HardwareSerial::begin() methods.

There are two begin methods, one which accepts just a baud rate and
uses the default bit settings and one which accepts both a baudrate and
a bit config. Previously, both of these contained a complete
implementation, but now the former just calls the latter, explicitely
passing the default 8N1 configuration.

Technically, this causes a small change: Before the UCSRC register was
untouched when calling begin(baud), now it is explicitely initialized
with 8N1. However, since this is the default configuration for at least
the Uno and the Mega (didn't check any others), probably for all avrs,
this shouldn't effectively change anything. Given that the Arduino
documentation also documents this as the default when none is passed,
explicitly setting it is probably a good idea in any case.

Author: matthijskooijman

hardware/arduino/avr/cores/arduino/HardwareSerial.cpp

@@ -283,44 +283,7 @@ HardwareSerial::HardwareSerial(
 
 void HardwareSerial::begin(unsigned long baud)
 {
-  uint16_t baud_setting;
-  bool use_u2x = true;
-
-#if F_CPU == 16000000UL
-  // hardcoded exception for compatibility with the bootloader shipped
-  // with the Duemilanove and previous boards and the firmware on the 8U2
-  // on the Uno and Mega 2560.
-  if (baud == 57600) {
-    use_u2x = false;
-  }
-#endif
-
-try_again:
-  
-  if (use_u2x) {
-    *_ucsra = 1 << _u2x;
-    baud_setting = (F_CPU / 4 / baud - 1) / 2;
-  } else {
-    *_ucsra = 0;
-    baud_setting = (F_CPU / 8 / baud - 1) / 2;
-  }
-  
-  if ((baud_setting > 4095) && use_u2x)
-  {
-    use_u2x = false;
-    goto try_again;
-  }
-
-  // assign the baud_setting, a.k.a. ubbr (USART Baud Rate Register)
-  *_ubrrh = baud_setting >> 8;
-  *_ubrrl = baud_setting;
-
-  transmitting = false;
-
-  sbi(*_ucsrb, _rxen);
-  sbi(*_ucsrb, _txen);
-  sbi(*_ucsrb, _rxcie);
-  cbi(*_ucsrb, _udrie);
+  begin(baud, SERIAL_8N1);
 }
 
 void HardwareSerial::begin(unsigned long baud, byte config)